http://www.marxists.org/ contains a comprehensive list of resources related to Marx and Marxism

Books and Articles:

Immanual Wallerstein was the progenitor of world systems theory with his ground breaking three volumes: The Modern World-System: Capitalist Agriculture and the Origins of the European World-Economy in the Sixteenth Century, (1976); The Modern World System II: Mercantilism and the Consolidation of the European World-Economy, 1600-1750, (1980) The Modern World System III: The Second Era of Great Expansion of the Capitalist World-Economy, 1730-1840s, (1988), all published by Academic Press: New York.

The important essay ‘The Development of Underdevelopment’ in The Monthly Review (September, 1969) and the book Capitalism and Underdevelopment in Latin America (1969) both by Andre GunderFrank were also seminal pieces of work, explaining the relationships of exploitation between countries and regions, particularly the Americas.

The Underdevelopment of Development: Essays in Honor of Andre Gunder Frank, Sing Chew and Robert Denemark(eds.) (1996) Sage: London.